> The Avogadro number isn't a fundamental physics constant

That's a pretty arbitrary assertion. Who defines what "fundamental" is? The number of periods of <something> of Caesium atoms used to define the duration of one second doesn't sound very fundamental either.

In fact, the second and candela aren't choices aren't that fundamental, mostly reflecting how those are measured. All the others come from the speed of light, fundamental charge, Plank's constant, and gases constant; those are pretty fundamental to physics.

Avogadro's number is used to define the atomic units, what makes a lot more sense than pushing it into metric calculations.

The wikipedia page is great: https://en.wikipedia.org/wiki/Metric_system#Future_developme...

You seem to be assuming that the atom is fundamental. It's not. Why not base things off the mass of an electron instead?

What you want, is to go down to the smallest unit possible, which is apparently Planck's Constant.

It's an arbitrary physics constant.